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July
2010
guest
feature article
Coupling Rainwater
Collection with Living Roofs:
A Water
Quality Review
By David
Williams and Jon Kinder, Prairie Designs LLC
One
of the questions that frequently arises when discussing green
roofs with clients is the feasibility of collecting the runoff
from the roof for various uses in and around a building.
Often
this interest is focused on reducing the watering requirement
for the roof – after all, in drought prone areas, watering your
roof seems to be wasteful, right? If your desire is simply
an ecologically friendly roof, why not just install a white roof
that you do not have to water?
We will
not address the green roof vs. white roof debate in this
article, but we will explore the water quality implications of
collecting surplus rainwater for use in watering the roof system
when it needs it. This water could also be used around the
landscape, or potentially even in the building.
The latter option, using green roof runoff in buildings as
potable water, almost always raises eyebrows. Would you
want to be drinking water that just passed through soil?
We have
heard differing opinions on this topic, and were very interested
when we found a recent report from the Texas Water Development
Board entitled: Effect of Roof Material on Water Quality for
Rainwater Harvesting Systems (January 31, 2010). This
report is
available for free, and has some applicable findings to
our field.
As the
title mentions, the study centered on the effect that different
roof treatments have on the water quality of collected
rainwater. A survey of the most common roofing materials
in Texas was completed, and found that the three most common
were asphalt-fiberglass shingles, Galvalume® metal panels, and
concrete tiles.
Three 8
foot x 4 foot test roofs were constructed at the
Lady Bird Johnson (LBJ) Wildflower Center in Austin to test
these three roofing treatments. Some readers may already know
that the LBJ Wildflower Center is home to one of the first green
roof research plots in the state (Publisher's Note:
See the profile in The Greenroof & Greenwall Projects
Database
here), testing various commercial green roofing systems and
other cool roof technologies over the course of many a hot,
Texas summer.
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The LBJ Test Greenroof Plots; Photo
Courtesy of Steve Windhager, PhD.
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In
addition to monitoring runoff from these three new test roofs,
the team also decided to monitor runoff from one of the green
roofing modules on site, as well as a white roof from the same
study.
The
team followed best practices for collecting rainwater, fitting
all collectors with first flush systems to capture and sequester
the dirtiest water before allowing additional water to flow into
the two subsequent storage tanks. Such first flush systems
are a standard feature on most rainwater harvesting systems
because the initial surge of rainfall across the roof washes any
contaminants on the roof into the collection system.
Southern Illinois University that found elevated levels of Lead
and Cadmium (Cd) leaching sporadically from many common green
roof soils (Alsup et al. 2010).
The collected water from each treatment was tested for a number
of water quality indicators, which are discussed below.
Results
Conductivity
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Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for
Rainwater Harvesting Systems
|
Conductivity is the ability of a substance to conduct an
electric current, and it turns out that extremely pure water is
less conductive than water with dissolved salts or other
impurities floating around. This is because these impurities
tend to ionize while in solution, and ions (non-neutrally
charged particles) allow electric current to flow more freely
through a medium. The green roof used in this study
produced the most conductive water samples.
It is hard to draw a finite conclusion from this, except to say
that the collected green roof runoff had more impurities in it.
Impurities are not necessarily good or bad; highly conductive
water often simply has a more mineral taste to it. If the
Wildflower Center Staff had been using city or well water from
the Austin area to water this roof, it is possible that minerals
from that water were deposited into the green roof soil and have
been subsequently leeching out into the runoff.
Turbidity / TSS
Turbidity is one measure of how many suspended solids are
floating around in a fluid. These solids, in high enough
concentrations, will cause a clear fluid like water to appear
cloudy or opaque. One might expect that water passing
through soil would be full of suspended solids; shouldn’t some
of that dirt be washing out?
It turns out that the green roof runoff had the lowest turbidity
out of any roofing treatment tested:
“The lowest turbidity values were found in rainwater
harvested after the first flush from the green roof…an
indication that green roofs can effectively filter out
particles. It is important to note, however, that all roofs
yielded higher turbidity values than the 1 NTU maximum
recommended for potable use of harvested rainwater, which is
the same as the U.S. EPA’s guideline for filtered surface
water.”
The results were similar for Total Suspended Solids (the measure
of how much stuff collects on a filter when a sample of water is
poured through it); the green roof showed the lowest values.
Take home point: if you’re going to drink your collected
rainwater, you should first run it through a sediment filter to
remove suspended solids.
Nitrates
Nitrates in water come from a number of sources, and have
potential health implications especially for very young
children. While health effects in adults are mild and rare, the
presence of nitrates in water usually indicates anthropogenic
contamination of some kind in the water.
Because the water samples in this study were collected from roof
surfaces, they are less susceptible to the more common vectors
of nitrate contamination (fertilizer, human/animal waste).
Almost all of the samples were below the EPA limit, and the
green roof had the lowest values of all treatments tested.
Dissolved Organic Carbon (DOC)
DOC tests measure the amount of organic material in a solution.
Organic materials from plants and animals can break down to such
small sizes that they can become dissolved in water.
ecause DOC is derived from living organisms, it makes sense that
the water collected from the green roof had the highest levels
of DOC’s.
The big implication here is on the use of chlorine to disinfect
collected rain water. Chlorine reacts with DOC to form all
sorts of not-so-good-for-you byproducts. To avoid this
problem all together, collected rainwater from a green roof
should be treated with a disinfectant other than chlorine before
use. UV light “filters” and ozone bubblers are proven
alternatives to Chlorine.
Coliform
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Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for
Rainwater Harvesting Systems
|
Runoff
from the green roof was lowest in both Total Coliform and Fecal
Coliform compared to the other treatments, but both were present
in all treatments after the first flush. Coliform can be
eliminated with a UV or ozone disinfectant system, but a green
roof goes a long way towards eliminating it before filtration.
Heavy Metals
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Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for
Rainwater Harvesting Systems
|
Roof
runoff was also tested for Aluminum (Al), Iron (Fe), Copper
(Cu), Zinc (Zn), Lead (Pb), and Chromium (Cr).
“For all rain events, rainwater harvested after the first
flush from the green roof consistently showed the lowest
concentrations of Al, Fe, Cr, and Cu…the highest Zn
concentrations were seen in the harvested rainwater after
the first flush from the green and metal roofs; elevated Zn
concentrations from the green roof might have been from the
solder in the scupper gutter.”
Additionally, the solder in the gutter was also a potential
source for the Lead found in the green roof runoff during one
rain event.
These are great results, but further study is needed to
determine if elevated Zinc and Lead levels were due to
contamination outside of the roof or not. If the gutters
are not to blame, this may corroborate a study conducted at
Southern Illinois University that found elevated levels of Lead
and Cadmium (Cd) leaching sporadically from many common green
roof soils (Alsup et al. 2010).
This question highlights the need for further data on
manufactured green roof soils and their propensity to leach
heavy metals, specifically Lead and Cadmium. Additionally,
it is always smart to consider what materials your
potentially-potable harvested rainwater flows across or through
on its way to your tanks; many roofing/guttering materials
contain these heavy metals.
Considerations
While this study is very informative, it only analyzed a small
number of rain events on small test roofs. Furthermore,
the green roof and white roof were almost flat, while the other
three treatments were inclined at just over 18 degrees.
The team also found extreme variability when their analysis were
scaled up to full sized residences located in different areas of
the city, suggesting that geographical location plays a role in
determining what ends up in your collection tanks.
Regardless of the source of your rainwater, you should always
follow the best practices when designing and installing your
system to prevent contamination. In association with
others, the Texas Water Development Board has produced "The
Texas Manual on Rainwater Harvesting," a comprehensive
manual that is available free, and considered by many to be one
of the best resources on the subject.
Conclusion
So how does this study affect our thoughts about harvesting
rainwater from a green roof?
It would seem that most harvested rainwater can easily be made
safe to drink if the levels of heavy metals are not too high.
In many cases, runoff from a green roof enters the
filtering/disinfecting process cleaner than water harvested from
many popular roofing alternatives – especially with respect to
heavy metals.
One should keep in mind that the volume of rainwater collected
from a green roof will be less than the volume collected from a
less absorbent roofing material. Many studies suggest that
around 60%-80% of the raindrops that fall on a green roof will
never make it into a collection tank, thus reducing the roof’s
effectiveness at producing storable water. This fact can
be both positive and negative, depending on project priorities.
Because less water is captured per square-foot from a green roof
vs. a metal or shingle roof, green roofs can use smaller,
cheaper water storage systems. In addition, if stormwater
management is a primary project goal, combining rainwater
collection with living roofs can be a great tactic. Doing
so will effectively increase the roof’s capture efficiency to
nearly 100% while providing a sustainable source of irrigation
water for the roof during the summer months. It is worth
noting that deciding against a green roof may be the right call
if a project or client needs larger quantities of captured
water; it is especially important that consultants be well
versed in both technologies to help a client make the right
decision.
While using collected green roof rainwater for potable purposes
still may be a few years away, the benefits of collecting green
roof runoff for landscape watering applications have inspired
multiple green roof projects to implement rainwater harvesting
systems. Here are a few:
• The high-profile
Beddington Zero Emission Development (BedZed) – Beddington,
UK
•
Circular Congregational Church - Charleston, SC
•
251 East Medical Center - Houston, TX
•
The Solaire - 20 River Terrace - New York, NY
•
Southface Eco Office - Atlanta, GA
•
901 Moreland Avenue (Eden House) - Atlanta, GA
David Williams and Jon Kinder
Further Reading
The Texas Manual on Rainwater Harvesting
Texas A&M Online Rainwater Resources
Rainwater Harvesting for Drylands and Beyond, Volume 1
References:
Alsup, S., Ebbs, S., & Retzlaff, W. (2010). The exchangeability
and leachability of metals from select green roof growth
substrates. Urban Ecosystems, 13(1), 91-111.
doi:10.1007/s11252-009-0106-y
Mendez, C. B., Afshar, B. R., Kinney, K., Barrett, M. E., &
Kirisits, M. J. (2010). Effect of Roof Material on Water
Quality for Rainwater Harvesting Systems. Texas Water
Development Board: P.O. Box 13231, Capitol Station Austin, Texas
78711-3231.
The Texas Manual on Rainwater Harvesting, Third Edition.
(2005). Retrieved from
https://www.twdb.state.tx.us/RWPG/rpgm_rpts/2003483510_RainwaterHarvesting.pdf
Dave Williams, LEED AP and Jon Kinder, LEED AP formed
Prairie Designs in 2009 after completing green roof research
at Texas Christian University in Fort Worth. Their research on
adapting green roofs for Texas was integral in the development
of the living roof on the new headquarters of the Botanical
Research Institute of Texas.
Prairie Designs' primary focus is creating drought-tolerant
living roofs based on native ecosystems. Additionally, they
manufacture Roof Rocks™, a patent-pending mulching system that
drastically reduces moisture loss in green roofing systems.
To learn more about Prairie Designs, please contact Dave
Williams at
dave@prairiedesignsllc.com and
Jon Kinder at
jon@prairiedesignsllc.com or call: 512.827.ROOF (7663).
Past Guest Feature
Articles
The opinions expressed
by our Guest Feature writers and editors may not necessarily reflect
the beliefs of Greenroofs.com, and are offered to our readers to
simply present individual views and experiences and
open
a dialogue of further discussion, debate and research. Enjoy,
and if you have a particular comment, please contact the author
or send us an email to:
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